Objective: To study the penetration of mammalian cells by potential chemotherapeutically useful nucleotides, normal and cyclic nucleotides, sugar phosphates and tRNA, and to investigate the effect of such cellular penetration on the viability of mammalian cells in culture and ascitic tumors in mice. Approach: The toxicity of arabinosyladenine 5'-phosphate (araAMP), 3',5'-cyclic arabinosyladenine monophosphate (3',5'-cyclic araAMP), 2',5'-cyclic arabinosyladenine monophosphate, arabinosylhypoxanthine 5'-phosphate, arabinosyladenine N1-oxide-5'phosphate and 2', 3'-dideoxyadenosine 5'-phosphate (ddAMP) to mouse fibroblasts in suspension culture was determined by assaying the number of viable cells after incubation with these compounds. The uptake and metabolism of (H3, P32)araAMP by fibroblasts was studied in detail, noting incorporation into nucleotides, RNA and DNA. Similar studies are being carried out on (H3, P32)AMP and (H3, P32)dAMP.